Scratch

1. It is not against the Scratch Terms of Use (not Service) To mod the Scratch Non-source. But it is against the Scratch ToU to have a share menu with the option of sharing to the website. To mod the Scratch Application (or EXE, because I use a Mac) You SHIFT+CLICK (the act of pressing shift, and clicking at the same time) the loop in the "R" in the Scratch logo at the top-left corner.

2. I don't really get what you are saying. To change the default sprite, it depends on if the Scratch Skin is in the modifications file on your computer. If it is, you replace "defaultSpriteCostume.gif" If it isn't, you make a sprite on Scratch right-click on it, chose "export this sprite" and drag the sprite file on your computer into the folder, of which you keep your modification.

3. BlockDefs? What the heck are BlockDefs? Do you even have the source code? It's

B-L-O-C-K-S-P-E-C-S

And, that isn't the full story of how to do it. You follow this "pathway"

Scratch-Objects

ScriptableScratchMorph (ScriptableScratchMorph is for the blocks that sprites and stage share, to edit things like motion blocks, because those are only on sprites, you go to ScratchSpriteMorph, and vice versa with ScratchStageMorph)

(class)

Block specs

blockSpecs

and edit freely. But if you want your blocks to actually do something, then instead of going to (class) you go to (instance) then pick the most logical given category, then you add the code. But if you are new to the source, look around and see how other blocks work to get the hang of doing it on your own. And... Oh, it seems as though I've already answered your question. 

Oh yeah, and don't edit the blocks, you add new ones.